It is clear that as the 21st century arrives the remarkable revolution in information services has permeated our society. In the past, communications confined to narrowband voice signals demanded a high quality visual, audio, and data context. Every aspect of human interplay of business, entertainment, government, or academia, depends on rapid and reliable communication networks. Indeed, the advent of the Internet alone is introducing millions of individuals to a new world of information and technology. The telecommunications industry, however, is struggling to keep pace with these changes.
Digital transmission equipment currently being deployed uses optical fibers to carry a single digital signal per fiber per propagation direction. The most successful and widespread type of digital optical protocol is synchronous optical network/synchronous digital hierarchy (SONET/SDH). Most high-speed digital backbones are primarily SONET/SDH-based.
SONET is an optical interface standard that allows interworking of transmission products from multiple vendors. SONET provides framing as well as a rate hierarchy and optical parameters for interfaces ranging from 51 Mb/s (OC-1) up to 9.8 Gb/s (OC-192). It defines a physical interface, optical line rates known as Optical Carrier (OC) signals, frame format, and an OAM&P (Operations, Administration, Maintenance, and Provisioning). SONET has been adopted by the ITU-T (International Telecommunication Union-Telecommunications Standardization Sector). The ITU-T version is known as SDH (Synchronous Digital Hierarchy). SONET/SDH networks typically are deployed in a ring physical topology, with multiple fibers providing redundancy. In addition, SONET may be deployed in a linear physical topology.
SONET was designed to provide a standard access to the optical transmission medium. It uses a specific frame format to carry data plus overhead bytes. SONET channels are synchronous. The synchronization of channels is supported by pointers, which dictate the initial byte position of each channel within the SONET frame. These pointers are used to multiplex digital signals within a single SONET frame efficiently.
The widespread use of fiber is possible, in part, by the industry's acceptance of SONET and SDH as the standard for signal transmission. Using SONET/SDH standards, telecommunication companies have gradually expanded their capacity by increasing data rates, to the point that many carriers now routinely transport 2.4 Gb/s (STM-16/OC-48).
However, once seemingly inexhaustible capacity promised by ever increasing SONET rates is reaching its limit. In fact, bandwidth demand is already approaching the maximum capacity available in some networks. Primarily because of technical limitations and physical properties of embedded fiber, there is a practical ceiling of 2.4 Gb/s on most fiber networks, although there are instances where STM-64/OC-192 is deployed. Surprisingly, the TDM (Time Division Multiplexing) equipment installed today utilizes less than 1% of the intrinsic capacity of the fiber!
The introduction of the wavelength-division multiplexing (WDM) provides more transmission capacity. WDM promises to multiply the bandwidth capacity of the optical transmission medium many folds. The principle behind this increase of the bandwidth capacity is the transmission of multiple digital signals using several wavelengths so there is no interference among the signals. Thus, DWM, effectively provides much greater bandwidth capacity of optical fibers.
WDM enables transmission of various optical signals by a single fiber. The principle behind WDM is essentially the same as the principle behind frequency-division multiplexing (FDM). That is, different carriers occupying non-overlapping parts of a frequency spectrum transmit different signals. In the case of WDM, the spectrum bands used are in the regions of 1310 and 1550 nm, which are the two wavelength windows where single mode optical fibers have very low signal loss.
Initially, a single optical signal transmits through each window. With the advance of optical components, such as distributed feedback (DFB) lasers, erbium-doped fiber amplifiers (EDFAs), and photodetectors, it was soon realized that several optical signals could be transmitting through each window, each optical signal occupying a small fraction of the total wavelength window available. In fact, the number of optical signals multiplexed within a window is limited only by the precision of these components. With current technology, over 100 optical channels can be multiplexed into a single fiber. The technology was then named dense wavelength-division multiplexing (DWDM).
The introduction of WDM or DWDM brings a variety of optical equipment into a central office. FIG. 1 depicts a SONET (Synchronous Optical NETwork) architecture 10, where optical fibers form a ring 12 connecting multiple Add/Drop Multiplexers (ADMs) 14. ADM 14 is also known sometime as Optical Add/Drop Multiplexers (OADMs). A ring 12, which can be a backbone ring, can be connected to another ring 12, which can be an access ring, through a connection from an ADM 14 to a digital cross-connect switch (DCS) 16. An ADM 14 can also be connected to another ADM 14. The connections between ADMs and between an ADM and a DCS can be done through fiber optical cables. Fiber optical cables are connected to a SONET through line terminating equipments (LTE) 18, which can be connected to an ADM 14 or a DCS 16.
FIG. 1 depicts a SONET ring 12 with multiple fibers. The SONET ring 12 also comprises multiple ADMs (add/drop multiplexers) 14. ADMs 14 are SONET devices, which perform low-rate signal grooming into the high-speed SONET signals used in the ring 12. ADMs 14 can also drop SONET signals from a SONET bit stream.
Digital Cross-Connect switches (DCS) 16 are devices used to connect SONET rings 12 together. DCS 16 cross-connects signals across rings 12, providing multiplexing/demultiplexing and switching functions. DCS 16 performs functions similar to that of a normal voice switch, except that connections are typically set-up in advance of when the circuits are to be switched—not coincident with a call.
Line terminating equipment (LTE) 18 is usually reserved for end-user or local area network (LAN) equipment. LTE 18 includes network elements, which originate and/or terminate line (OC-N) signals.
Add/drop multiplexer (ADM) 14, which is connected to a SONET ring 12, adds or drops optical signals from the SONET ring 12. Incoming audio, video, and data signals are converted into optical signals and sent to their destinations through the SONET ring 12. Generally, the optical signals that are added to a SONET ring 12 for transport to their destinations or dropped from the SONET ring 12 for local processing and forwarding to the end user are handled in a central office (CO).
In FIG. 3, the optical signals dropped from an ADM 14 are terminated on the back panel of a six-pack connector located on a light guide cross-connect (LGX) patch panel, also known as LGX distribution frame 34. An optical connector 32 plugged into the front panel of the six-pack connector takes the optical signal to its destination equipment, which can be an optical receiver located on a different floor from the CO or a WDM multiplexer 36 located in a frame several aisles away or on a different floor.
Still referring to FIG. 3, often several optical signals are taken from a LGX patch panel 34 and merged into a single optical signal at a passive WDM multiplexer 36, such as a 16 channel WDM multiplexer from Lucent Technologies, located several floors away from the LGX patch panel 34, from where the merged optical signal is connected to an outgoing fiber optical cable 32. The outgoing fiber optical cable 32 carrying the merged optical signals is then routed back to the LGX patch panel 34 and connected to the back panel of a six-channel connector.
The connection between the LGX patch panel 34 and the passive WDM multiplexer 36 can be several hundred feet away, and the connection cables 32 are generally routed through overhead racks. When a new fiber optical signal is to be added to the outgoing fiber optical cable 32, a new work order needs to be issued for routing new connection optical cables between the LGX patch panel 34 and the passive WDM multiplexer 36.
The resulting installation procedure is costly and time consuming, and the lengthy connecting fiber optical cables carrying these optical signals can degrade the quality of these optical signals.
Another problem created by the introduction of WDM or DWDM in optical networks is an increase in the cost of spare parts. Optical signals are generated by optical transmitters, which take electrical digital signals and convert them into optical signals of a specific frequency. Each optical transmitter is generally designed for a specific optical frequency. In a CO that generates 16 different narrow band optical signals, the CO needs to carry spare parts for each one of the 16 transmitters.